1. Field of the Invention
The present invention relates to a hexagonal ferrite magnetic powder, a method for producing the same and a magnetic recording medium. More particularly, it relates to a hexagonal ferrite magnetic powder which provides good output characteristics and yields a reduced noise and which is adapted for a magnetic recording medium for high-density recording reproducible by a highly sensitive head such as an MR head or a GMR head. Also, the invention relates to a magnetic recording medium containing the hexagonal ferrite magnetic powder in the magnetic layer.
2. Description of the Related Art
In the field of magnetic recording, a magnetic head based on the principle of electromagnetic induction (induction type magnetic head) has been used and spread. However, the induction type magnetic head is becoming insufficient for use in the presently required reproduction of high-density records. That is, when the number of coil turns of a reproduction head is increased in order to obtain a larger reproduction output, there results an increased inductance and an increase in resistance in high frequency region, leading to a problem of reduction in reproduction output. Thus, in recent years, a reproduction head based on the principle of MR (magnetic reluctance) has been proposed and used for a hard disc or the like. The MR head yields a reproduction output several times as much as that of the induction type magnetic head and, since no induction coils are used, device noises such as impedance noise are markedly reduced. Therefore, a large SN ratio can be obtained.
On the other hand, the improvement of high-density recording characteristics can also be made by reducing magnetic recording medium noise having conventionally been hidden behind the device noises.
In order to attain such object, there has been proposed, for example, a magnetic recording medium comprising a non-magnetic support having provided thereon a magnetic layer containing a hexagonal ferrite magnetic powder dispersed in a binder (see, for example, JP-A-10-312525).
Also, the improvement of the hexagonal ferrite magnetic powder is disclosed in the following JP-A-10-92618 and Japanese Patent No. 1,652,829. The JP-A-10-92618 discloses a hexagonal ferrite magnetic powder having an average particle size of from 30 to 40 nm, a specific surface area measured by the BET method of from 30 to 70 m2/g, an SFD of 0.9 or less, and a probability of existence of particles having an average article size of 10 nm or less of 5% or less. Also, the Japanese Patent No. 1,652,829 discloses a method of producing a hexagonal ferrite according to the glass crystallization method, which comprises heating the temperature at a rate of 200° C./hr in the step of heating an amorphous product at a temperature of 775 to 800° C. to crystallize hexagonal ferrite to thereby depress the maximum particle size to 0.3 μm or less.
However, these related techniques have failed to provide a magnetic recording medium adapted for a high-density recording as high as about 1 Gbpsi, which is a presently required level.